I always wandered this myself. My thought was that maybe he wasn't being re-animated by the same forces as the other ghosts? IE; Paranormal forces? It also balances him out a little. To have an enemy toting two pistols AND a highly damaging aura? A little OP.

I thought about it too. Maybe Walter is resurrected by different means...

But Joseph (15/21) had neither damaging aura. When Henry meets him in his room, he even talked to him. But the whole room contains lit holy candles, so maybe it keeps the effects of the ghost aura at bay...

If you count the "angry ghosts" thing, Walter didn't had any hate (though he demonstrates guilt when he gets hauled to jail) because he died believing he was working for the greater good (he was going to see his "mother" again and shut himself inside a confortable world). Thus, no miasma and/or aura.

Plus, the name of his particular sacrifice tells everything we need to know: Assumption. Guy became a dark Buddha.

But again, Valtiel might have something to do with the "special resurrection" since he is, you know, "part" of Walter...